Project Summary The long-term goal of this project is to identify an appropriate preclinical mouse model to characterize the etiology of Hereditary hemorrhagic telangiectasia (HHT) and to develop novel treatment options for the disease. HHT is a genetic disorder with arteriovenous malformations (AVMs) as a core pathology throughout the body. AVMs are abnormal direct connections from arteries to veins without intervening capillaries. Symptoms of HHT include telangiectasias, epistaxis, severe headache, epilepsy, stroke, gastrointestinal bleeding, and AVMs in brain, lung, and liver. The worldwide estimated incidence of HHT is 1:50002, but 90% of people with HHT are undiagnosed. Currently, there is no prevention or cure for the major clinical manifestations of HHT, calling for the development of better treatment strategies. The TGF? type I receptor activin A receptor-like type 1 (ACVRL1 or Alk1) gene has been implicated in the etiology of HHT type 2 (HHT2), and mutation of this gene is estimated to underlie 25- 57% of all HHT cases3?7. Alk1 is primarily expressed in endothelial cells and is essential for vascular development8?10. Previous mouse models of HHT2, based on deletion of Alk1, exhibited certain disease aspects11?14. However, the existing animal models are not ideal animal models for HHT, as animals either die early by postnatal day 5 (homozygous deletion) or develop vascular lesions with long latency and incomplete penetration (heterozygous deletion) lacking key HHT symptoms, e. g. nosebleed15. Therefore, there is a critical need to develop a better preclinical animal model that better reflects the vascular lesions presented in HHT2 patients, aiding the efforts to uncover the mechanisms of the disease progression and develop potential preventative and treatment strategies for HHT therapy. To resolve these gaps in knowledge, our lab has created a novel mouse model in which Alk1 is deleted postnatally in arterial endothelial cells. Preliminary data suggest that these mice survived beyond postnatal day 27 with HHT-like vascular lesions in multiple organs, offering a valuable animal model of HHT2 to study the disease processes and underlying mechanisms. I will characterize the HHT-like lesions present in these mice by examining the gross pathology, histopathology, animal health, and behavior of the animals. Further, I will examine the vascular structure and function through microscopic imagining, including two-photon in vivo live imaging. Finally, I will characterize the oxidative stress profile of the mice by measuring superoxide and NO levels, while also treating the mice with a superoxide dismutase mimetic, TEMPOL and a synthetic BH4 compound, sepiapterin to determine the effects of oxidative stress in the formation and prevention of AVMs in this model. The successful completion of this project will provide a novel tool for the investigation of mechanism underlying Alk1-mediated HHT formation and for the development of novel preventative and curative therapies for patients with HHT2.